Method and circuit for row scannable latch array

ABSTRACT

Testability of memory on integrated circuits is improved by connecting storage elements like latches in memory to scan chains and configuring memory for scan dump. The use of latches and similar compact storage elements to form scannable memory can extend the testability of high-density memory circuits on complex integrated circuits operable at high clock speeds. A scannable memory architecture includes an input buffer with active low buffer latches, and an array of active high storage latches, operated in coordination to enable incorporation of the memory into scan chains for ATPG/TT and scan dump testing modes.

PRIORITY APPLICATION

This application claims the benefit of U.S. Provisional Patent Application No. 63/107,413 filed 29 Oct. 2020, which application is incorporated herein by reference.

FIELD OF THE TECHNOLOGY

The present technology relates to the field of integrated circuits including memory designed for testability and diagnostics including, in some embodiments, scan chain compatible memory testable using one or more of automatic test pattern generation (ATPG), transition testing (TT) and memory built-in self-test (BIST).

BACKGROUND

Integrated circuits include various types of on-chip memory circuits. Examples of such memory circuits which can be used in critical processing paths are known by such names as working memory, cache, buffers, registers, First-In-First-Out buffers (FIFOs), Look-Up-Tables (LUTs), Least Recently Used (LRU) buffers, and various types of queues. In many settings, memory circuits must be stable and fast, and can occupy significant area on a chip.

Integrated circuits often incorporate structures to support testing of the circuits on the devices, using Design for Testability (DFT) techniques. One aspect of such techniques includes implementation of scan chains on the integrated circuit, which connect flip-flops and registers used in the functional circuit, to form a serial chain parallel to the functional circuit paths of the device. Data patterns can be loaded in the chip using the scan chain, the functional circuits can be exercised, and then the resulting pattern read out using the scan chain for analysis. An early example of such a technique is a scan design known as Level Sensitive Scan Design (LSSD) described in U.S. Pat. No. 3,761,695 to Eichelberger.

It is desirable to provide technologies for efficient integrated circuit memory with improved testability.

SUMMARY

Technologies are described herein which improve the testability of memory on integrated circuits supporting, in various embodiments, connecting storage elements like latches in memory to scan chains and configuring memory for scan dump. The use of latches and similar compact storage elements to form scannable memory can extend the testability of high-density memory circuits on complex integrated circuits operable at high clock speeds.

A scannable memory circuit is described for implementation on an integrated circuit having a scan chain, in one aspect of the technology herein, in which the memory is composed of an array of storage elements, such storage latches, having a plurality of rows and a plurality of columns. Also, the circuit includes an input buffer connected to the array of storage elements, including a row of buffer latches enabled to load data during a first part of an input clock signal, such as having active low enable inputs connected to the input clock. The storage elements in the array are enabled to load data during a second part of an input clock signal, such as by having active high enable signals coupled to a row clock signal synchronized with the input clock. The row of buffer latches is configured to transfer data during the second part of the input clock signal from the input buffer to storage elements of a selected row in the plurality of rows in parallel. The memory circuit in this aspect of the technology includes an output selector connected to the array of storage elements having inputs connected to the storage elements in the array of storage elements, to output data from storage elements in a selected row in parallel to an output bus having lines DOUT[N−1:0]. To support inserting storage elements in the array in a scan chain, an input data selector having an output DIN[N−1:0] is connected to the row of buffer latches in the input buffer and selects, in dependence on a scan enable signal, from a first input connected to a functional path data, and a second input connected to a scan mode input bus. The scan mode input bus has a scan-in line connected to the scan chain for connection to DIN[N−1] and a plurality of feedback lines for output data DOUT[N−1:1] from storage elements in the selected row for connection to DIN[N−2:0], respectively. Also, the memory circuit includes a circuit, responsive to the scan enable signal to connect output data DOUT[0] to a scan-out line connected to the scan chain.

To support testing such as automatic test pattern generator ATPG scan tests and transition testing using ATPG, a control circuit can be included to provide the scan enable signal, and to set a row address to the selected row during a scan operation shifting data from the scan-in line through the selected row to the scan-out line. The control circuit can be configured to execute a procedure including: setting the row of input selectors to a scan mode; setting the row enable signal to a fixed row in the scan mode; sequentially while in a scan mode, loading during the first part of the clock period a bit from the scan chain in the first buffer latch of the input row and a bit from the output line of the next adjacent column in the other latches in the input row, and writing during the second part of the clock period the bits in the input row of latches to the storage elements in the fixed row in the corresponding columns; and applying the line on the output bus corresponding to the last column of the array to a scan-out line.

A scannable memory circuit is described for implementation on an integrated circuit having a scan chain configured for scan dump (a scan dump chain), in one aspect of the technology herein, in which the memory is composed of an array of storage elements having a plurality of rows and a plurality of columns. An input buffer in this aspect of the technology is connected to the array of storage elements, including a row of buffer latches enabled to load data during a first part of an input clock signal, and configured to transfer data during a second part of the input clock signal from the input buffer to storage elements of a selected row in the plurality of rows in parallel. An output selector is connected to the array of storage elements having inputs connected to the storage elements in the array of storage elements, to output data from storage elements in a selected row in parallel to an output bus having lines DOUT[N−1:0]. An input data selector having an output DIN[N−1:0] connected to the row of buffer latches in the input buffer selects, in dependence on a scan dump signal, from a first input connected to a functional path data, and a second input connected to a scan dump input bus, the scan dump input bus having a plurality of feedback lines for output data DOUT[N−1:1] from storage elements in the selected row for connection to DIN[N−2:0], respectively. The line of the scan dump input bus for connection to DIN[N−1] can be disconnected or connected to a reference node in some cases, or in other cases connected to a scan dump in line to receive scan dump data from other circuits on the device. A circuit is provided, responsive to the scan dump signal, to connect output data DOUT[0] to a scan-out line connected to the scan dump chain.

To support testing operations including scan dump, a control circuit to provide the scan dump signal, and increment the selected row during a scan dump operation. As a result, the scan dump operations shifts data from the plurality of rows in sequence to the scan-dump out line. The control circuit can be configured to execute a procedure including: setting the row of input selectors to a scan mode; sequentially while the input clock is running in a scan dump mode, loading during the first part of the clock period a bit from the scan chain in a first buffer latch of the input row and a bit from the output line of the next adjacent column in the other buffer latches in the input row, and writing during a second part of the clock period the bits in the input row of buffer latches to the storage elements in a selected row in the corresponding columns; incrementing the row enable signals while the input clock is running in the scan dump mode by one row after a number of input clock periods equal to the number of columns of the array; and applying a line on the output bus corresponding to the last column of the array to a scan-out line.

These and other features, aspects, and advantages of the technology disclosed will become apparent from the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to like parts throughout the different views. In the following description, various implementations of the technology disclosed are described with reference to the following drawings.

FIG. 1 is a system diagram illustrating a scan chain and scan dump tester, MBIST data verification and an example reconfigurable processor.

FIG. 2 depicts an example four-row, four column scannable latch array.

FIG. 3 illustrates an example data bus multiplexer interposed between a memory built-in self-testing (MBIST) and a Latch Array.

FIG. 4 illustrates an example storage latch array row clocking circuit.

FIG. 5 shows example address multiplexers for selecting rows in the array in a plurality of modes.

FIG. 6 shows example multiplexers for providing write enable and read enable signals in a plurality of modes.

FIG. 7 depicts a Latch Array in the ATPG row “0” scannable mode using “A” latch row multiplexers.

FIG. 8 is an example of a testable Latch Array wrapper and pin-out.

FIG. 9A depicts an example Latch Array loading row “0” with serial-in data operative in Scan Dump mode.

FIG. 9B depicts an example Latch Array loading row “1” with serial-in data operative in Scan Dump mode.

FIG. 9C depicts an example Latch Array loading row “2” with serial-in data operative in Scan Dump mode.

FIG. 9D depicts an example Latch Array loading row “3” with serial-in data operative in Scan Dump mode.

FIG. 9E depicts an example Latch Array loading row “0” with serial-in data in Scan Dump mode and shifting a first row of data to scan-out.

FIG. 10A illustrates a four row, four column Latch Array after 17 scan shift clocks.

FIG. 10B illustrates a four row, four column Latch Array after 31 scan shift clocks.

FIG. 10C illustrates a four row, four column Latch Array after 32 scan shift clocks.

DETAILED DESCRIPTION

The following discussion is to enable any person skilled in the art to make and use the technology disclosed. Various modifications to the disclosed implementations will be clear to those skilled in the art, and the general principles defined can apply to other implementations and applications without departing from the spirit and scope of the technology disclosed. Thus, the technology disclosed is not limiting to the implementations shown but is to be accorded the widest scope consistent with the principles and features disclosed.

FIG. 1 is heuristic diagram of a data processing system including a tester 20, a memory built-in self-test MBIST module 140, a data verification module 30 included as part of, or coupled to, an integrated circuit 40 which includes one or more scannable storage element arrays 102. As illustrated, the circuit 40 can include scannable storage element arrays configured as scalar FIFOs 17 and Vector FIFOs 16. Other types of memory can be included on the integrated circuit, implemented using scannable storage element arrays as described herein. In this example, the integrated circuit 40 comprises a reconfigurable processor (RP) having an array of reconfigurable units (e.g., PCU 41, PMU 42), the units including or being coupled to the scannable storage element arrays 102 for various purposes. In one embodiment, the integrated circuit comprises a coarse grain reconfigurable array (CGRA) of reconfigurable units, such as described in Prabhakar et al., “Plasticine: A Reconfigurable Architecture for Parallel Patterns,” ISCA'17, Jun. 24-28, 2017, Toronto, ON, Canada, which is incorporated by reference as if fully set forth herein. Scannable storage element arrays as described herein can be part of any type of integrated circuit, such as a microprocessor, a digital signal processor, a microcontroller, an application-specific integrated circuit (ASIC), a communications device, a memory controller, a field programmable gate array (FPGA), or the like.

The Tester 20 comprises an automatic test pattern generator (ATPG), transition testing (TT), scan dump and scan chain interfaces, scan logic and scan verification. The Tester outputs MODE_SELECTS 21 to choose between the functional mode, the MBIST mode, scan dump mode and the ATPG/TT scan modes. A SCAN_IN line 130 injects serial test data to a scan chain on the integrated circuit 40. The SCAN_CONTROL 22 is a bus comprising the scan shift clock. The SCAN_OUT/SCAN_DUMP OUT line 160 is the data output after traversing the scan chains in the RP. The MBIST 140 outputs MBIST_ADDR/DATA (line 24) which comprise the address, data, and enable lines to test the Latch Array rows. The MBIST 140 also outputs the MBIST_COMPARE 146 vectors to the Data Verification module 30. The Data Verification module 30 contains the Data Out Circuits and Comparators 190 whose first input is the vector DOUT [N:0] 34 which must compare with MBIST_COMPARE 146 vectors. The DOUT [N:0] 34 is the output generated by the MBIST controller. In some embodiments, the MBIST 140 and Data Verification module 30 are embedded within the example integrated circuit 40 block.

As described herein, some or all memory implemented using scannable storage element arrays on the integrated circuit are incorporated in one or more scan chains as represented by line 25 (and/or scan dump chains which can be scan chains configured to support scan dump), and testable using at least one of the tester 20, Memory Built In Self Test MBIST module 140 and data verification module 30, and some or all memory implemented using storage element arrays on the integrated circuit can be written with data from line 24 and output data on line 34 for use with the MBIST unit 140.

FIG. 2 is schematic diagram of an implementation of a scannable storage element array in which the storage elements are latches. The array of FIG. 2 includes four rows by four columns (4×4) of latches. The scannable array comprises one row 104 of “B” latches B0 to B3 (“buffer latches”), and four “A” latch rows, a row 106 of “A” latches (“storage latches”), a row 107 of “A” latches, a row 108 of “A” latches, a row 109 of “A” latches. The “B” latches in this example are enabled “active low” in response to the clock on line 215, and latch input data from the corresponding multiplexer during the interval when the clock on line 215 is low. The “A” latches in this example are enabled “active high” in response to the corresponding row clock on line 126 b, and latch respective bits of input data LA_DIN[3:0] from “B” latches in respective columns during the interval when the corresponding row clock on line 126 b is high.

The “A” and “B” latches are storage elements having enable inputs which pass their input data to their output when the enable signal is asserted, and hold the last value of the input data when the enable signal is de-asserted. Because they pass their input data to their output when the enable signal asserted, they can be referred to as transparent latches. As a result, their output is not necessarily stable during the time that the enable signal is asserted, and depends on the fluctuations of the input signals in that enabled interval.

The “A” and “B” latches in scannable latch array of FIG. 2 can be implemented using D-latches or other storage element circuits, with active high and active low enable inputs, respectively. In this example, the enable input to the each of the “B” latches is active low, and so the “B” latch is enabled when the CLOCK signal is low (a “second part of the clock period”), and holds the data value on its input at the end of the low phase of the clock during the time that the CLOCK signal is high (a “first part of the clock period”), that is during the high phase of the CLOCK signal. Also in this example, the enable input to each of the “A” latches is active high, and so the “A” latch is enabled when the corresponding row clock signal (e.g. CLK[0]) is high, and holds the data value on its input at the end of the high phase of the corresponding row clock signal (e.g. CLK[0]), that is during the high phase of the corresponding row clock signal (e.g. CLK[0]).

In this embodiment, the CLOCK signal and the row clock signals (e.g. CLK[0]), are synchronized, meaning herein that the high and low phases are aligned sufficiently for this operation to operate without unacceptable glitches. So the “B” latch captures an input value at the end of the low phase of the CLOCK signal, which corresponds with the end of the low phase of the row clock signals, and holds the captured input value during the high phase of row clock signals which corresponds to the high phase of the CLOCK signal. This relationship establishes a master-slave flip-flop operation, in which the “B” latch is a master latch for a corresponding column of “A” latches, and the “A” latch of a selected row is the slave latch. The “A” latch in a selected row therefore latches new data at the rising edge of the row clock signal. Because the master “B” latch is holding its data during the high phase of the CLOCK signal, and the slave “A” latch changes only during the high phase of the row clock signal, any changes in data in the latch array are synchronized the leading edge of the row clock signal.

More generally, the “B” latches are enabled in a first part of a clock period of a scan clock, and the “A” latches are enabled in a second part of the clock period, so that data is captured in the “A” latch on transition from the first part to the second part of the clock period. Data read from the latches is stable during the second part of the clock period. For example, in an alternate embodiment, the “B” latches can be active high, and the “A” latches can be active low.

An “A” latch in the latch array can be incorporated into a scan chain that utilizes this master-slave operation. The use of transparent latches in the array (“A” latches) results in substantial saving of area because the circuits to implement them are smaller than circuits for flip-flops. The configuration described herein however, provides the ability to include storage elements in the array in a scan chain, and to support other testing methodologies.

In the row 104 of “B” latches, input multiplexers 230-233 are connected to the inputs of respective “B” latches B0-B3, and have control inputs connected to a scan enable signal SCAN_EN on line 290. The outputs of the input multiplexers 230-233 collectively provide a scan mode input bus and/or a scan dump input bus (not separately shown in the figure). Input multiplexer 233 has its output connected to the input of “B” latch B3. A first input of the input multiplexer 233 is a corresponding line from the data in bus 120 b, carrying bit 3 of the data DIN[3:0], and a second input of the input multiplexer 233 is a SCAN-IN signal received from a scan chain.

Input multiplexer 232 has its output connected to the input of “B” latch B2. A first input of the input multiplexer 232 is a corresponding line from the data in bus 120 b, carrying bit 2 of the data DIN[3:0], and a second input of the input multiplexer 232 is a corresponding line from a data out bus 34, carrying bit 3 of the data signal DOUT[3:0].

Input multiplexer 231 has its output connected to the input of “B” latch B1. A first input of the input multiplexer 231 is a corresponding line from the data in bus 120 b, carrying bit 1 of the data DIN[3:0], and a second input of the input multiplexer 232 is a corresponding line from a data out bus 34, carrying bit 2 of the data signal DOUT[3:0].

Input multiplexer 230 has its output connected to the input of “B” latch B0. A first input of the input multiplexer 230 is a corresponding line from the data in bus 120 b, carrying bit 0 of the data DIN[3:0], and a second input of the input multiplexer 230 is a corresponding line from a data out bus 34, carrying bit 1 of the data signal DOUT[3:0].

A first mode of operation is the functional mode in which the multiplexers 230 to 233 connect corresponding lines of the input bus carrying DIN[3:0] to the inputs of latches B3 to B0. A second mode of operation is the scan mode, where the multiplexer 233 connects SCAN-IN on line 130 to the input of latch B3, the multiplexer 232 connects DOUT[3] from bus 34 to the input of latch B2; the multiplexer 231 connects DOUT[2] from bus 34 to the input of latch B1; and the multiplexer 230 connects DOUT[1] from bus 34 to the input of latch B0. During an MBIST mode, the input bus carrying DIN[3:0] can carry the MBIST input data.

Also, included is an “A” latch row clocking circuit 126. The latch row clocking circuit 126 receives the clock on line 215, and row select signals (e.g., addresses) not shown, and applies the clock signal from line 215 to a selected row. In some embodiments, the clock signals applied on the selected rows can be generated from alternate sources, so long at the operative timing is correct.

Also, are “A” latch row select multiplexers 260 to 263 (in group 110) having inputs connected to the outputs of one “A” latch in each row in respective columns of the array. The control signal READ-ADDR on line 256 is connected to “A” latch row multiplexers 110 and selects an “A” latch row to couple onto DOUT [3:0] 34.

During a write operation for both functional and scan modes, the selected one of the inputs SCAN-IN or DIN [3] for multiplexer 233 is input to the corresponding “B” latch, B3. The B3 latch will output LA-DIN [3] to a first column of “A3” latches. Decoding a physical address causes latch row clocking circuit 126 to provide a valid row clock for the A3 latch of the selected “A” latch row, the A3 latch of the selected row captures the data while the row clock is high. In this example, there are four row clocks for the four “A” latch rows.

Similarly, during a write operation for both functional and scan modes, the selected one of the inputs DOUT [3] or DIN [2] for multiplexer 232 is input to “B” latch, B2. The B2 “B” latch will output LA-DIN [2] to a second column of “A2” latches, and the A2 latch of selected the row captures the data while the row clock is high.

Similarly, during a write operation for both functional and scan modes, the selected one of the inputs DOUT [2] or DIN [1] for multiplexer 231 is input to “B” latch, B1. The B1 “B” latch data will output LA-DIN [1] to a third column of “A1” latches, and the A1 latch of the selected row captures the data while the row clock is high.

Similarly, during a write operation for both functional and scan modes, the selected one of the inputs DOUT [1] or DIN [0] for multiplexer 230 are input to “B” latch, B0. The B0 “B” latch data will output LA-DIN [0] to a fourth column of “A0” latches, and the A0 latch of the selected row captures the data while the row clock is high.

For a write operation in the functional mode, the latch array can store input vectors in parallel. In the example illustrated, the input vector is four bits DIN[3:0] received from a functional data path in the integrated circuit. In other embodiments, the input vector can have any width, such as eight bits, 16 bits, 32 bits, 128 bits, and so on. In a one cycle write of an input vector DIN[3:0], the bits of input data DIN[3:0] on the bus 120 b are captured in parallel in the “B” latches B3 to B0 while the clock signal on line 215 is low, and held while the clock signal is high. While the row clock signal on line 126 b of the selected row, e.g. CLK[0], is high, the data bits captured during the low phase and held during the high phase of the input clock signal on line 215 in the “B” latches B3 to B0 are captured in parallel in the corresponding “A” latches A3 to A0 of the selected row, and passed through to the multiplexers 263-262 and the output data bus 34 as bits DOUT[3:0]. The “B” latches B3 to B0 hold the data bits received at their respective inputs at the end of the high phase of the row clock while the corresponding row clock is low. In this example, the input clock on line 215 and the row clocks on lines 126 b are synchronized, meaning herein that the high and low phases are aligned sufficiently for this operation to operate without unacceptable glitches.

In the scan modes, input data captured while the input clock is low includes four bits: the SCAN-IN bit from a scan chain in latch B3, DOUT[3] in latch B2 to shift the data from latch B3 to latch B2, DOUT[2] in latch B1 to shift select data from latch B2 to latch B1, and DOUT[1] in latch B0 to shift the data from latch B1 to latch B0. While the row clock signal on line 126 b of the selected row, e.g. CLK[0], is high, the data bits captured during the low phase and held during the high phase of the input clock signal on line 215 in the “B” latches B3 to B0 are captured in parallel in the corresponding “A” latches A3 to A0 of the selected row, and passed through to the multiplexers 263-262 and the output data bus 34 as bits DOUT[3:0]. The bit DOUT[0] from the last column of the latch array is the SCAN_OUT bit for the latch array, and is fed to the scan chain SCAN_OUT line 160 through a buffer 234, in this example. This feedback from the output bus DOUT[3:0] shifted by one position implements a serial shift data path through the latch array, which is inserted into the scan chain of the integrated circuit.

Generally, the circuit of FIG. 2 is an example of a memory circuit in which the input row includes a number N of latches B(n), the input bus includes the number N of input bus lines DIN(n), and the output bus includes the number N of output bus lines DOUT(n), where N is the number of columns of the array. Also, the first input of the input selector connected to the data input of latch B(n) is connected to input bus line DIN(n), for n going for 0 to N−1, and the second input of the input selector connected to the data input of latch B(n) in the input row other than the first latch is connected to output bus line DOUT(n+1), for n going from 0 to N−2, that is to a next adjacent column in the array. The variable N can be any practical positive integer. The number N in this example corresponds with the number of columns in the array. Although in the examples illustrated, N is a power of 2, there can be any number of columns. Other implementations include numbers columns that are not powers of 2, and odd numbers of columns. Also, in the illustrated example the number of rows is a power of 2, there can be any number of rows. Other implementations include numbers of rows that are not powers of 2, and odd numbers of rows.

In another embodiment, the storage elements implemented using the “A” latches in the array of FIG. 2 can be implemented using SRAM bit cells, typically six transistor cells including opposing inverters and pass gates connecting the opposing inverter inputs to true and complement terminals of the cells, which are connected to corresponding true and complement bit lines. The bit lines in a scannable storage element array as described herein can be shared by the bit cells in each column. Sense amplifiers are coupled to the true and complement bit lines, which provide output data for a selected bit cell in the corresponding column. In this case, the multiplexers 260 to 263 can be replaced by the sense amplifiers for the corresponding columns of cells. Also, the LA_DIN[3:0] lines can be differential signals applied to the true and complement bit lines. The row clocks CLK[3:0] are applied to the word lines which connect to enable inputs of pass gates of the bit cells in corresponding rows. An SRAM bit cell can be implemented so that it is enabled when the word line signal is high (a second part of the clock period), and holds data while the word line signal is low (a first part of the clock period). In this functional sense, it is like a transparent latch and can be used for implantation of scannable arrays as described herein.

FIG. 3 is an example schematic illustrating a DIN[3:0] data path comprising data multiplexer block 120. The control MB_RUN on line 320 is a selector input to the data multiplexer block 120, which selects between memory built-in self-test data MBDIN (such as MIST DATA on line 24 of FIG. 1) and DATA from the functional circuitry represented by cloud 120 a. In this example, the multiplexer block 120 comprises four multiplexers 310-313. When the MBIST function is running the signal MB_RUN on line 320 causes the multiplexers 310-313 to select data MBDIN. When the MBIST function is not running, the signal MB_RUN on line 320 causes the multiplexers 310-313 to select data DATA. The data multiplexer block 120 outputs DIN [3:0] on bus 120 b which is applied to the multiplexers 263-260 of FIG. 2.

FIG. 4 is an example schematic illustrating an implementation of an array row clocking circuit 126 block suitable for use in the circuit shown in FIG. 2. This example schematic is an implementation of a clocking circuit that provides a decoded clock for each row. Referring to the details of the schematic, the output of the AND gate 416 is a row clock CLK[N] on line 126 b of CLK[3:0] shown in FIG. 2. The address S_WA [1:0] 328 are input to a decoder 430 (e.g. a one hot decoder) for row selection in the functional write mode. Control signals SCAN_EN 290 and S_WE 348 are inputs to an OR gate 422. The AND gate 432 receives a first input 423 from the output of the OR gate 422 and a second input 431 from the decoder 430 to provide a data input 433 to active low latch 434, which is clocked by the input clock on line 215 (or other synchronized clock), which is a scan clock in the scan modes and a functional clock in the MBIST and functional modes. The clock 215 and “B” latch 434 Q output 436 are input to the AND gate 416 to produce the corresponding row clock on line 126 b. Thus, the row address in the latch array is captured at the same time (e.g. low phase of the input clock on line 215) as the input data is latched in the “B” latch row of the latch array, and stable during the access to the selected row. Similar circuits can be used to produce the row select signals in response to read enable RA signals.

FIG. 5 is an example schematic showing an implementation of an address multiplexer circuit 122 supporting four modes of operation of the latch array, applying addresses for accessing the array. The first mode is the functional mode used in operation of the array. The second mode is a memory built-in self-test mode. The third mode is a scan dump mode, and the fourth mode is the scan mode usable for ATPG/TT and similar testing. The address multiplexer circuit 122 selects between the functional address bus, the MBIST generated address bus, the scan dump generated address bus, and the ATPG one-row row-0 mode. This example implementation shows two address scan multiplexer circuits; one multiplexer circuit is for write addresses, and one multiplexer circuit is for read addresses.

According to one embodiment, the example write address multiplexer has an output multiplexer 329 operative to output the write address S_WA [1:0] on line 328 for supply to the row clock circuit of FIG. 4, and to output the read address S_RA[1:0] on line 338, for generation of the read address controlling the output multiplexers 260-263 of FIG. 2. The first output multiplexer 329 receives inputs 381 from a second multiplexer 324 and input 327 from a third multiplexer 326. The output multiplexer 329 output uses SCAN_CTL 323 to determine modes of operation and to select which address to output. The second multiplexer 324 selects between the functional input write address wa[1:0] from logic 122 a and MBIST generated write address mbwa[1:0] from line 142 a, and is controlled by the MB_RUN control signal on line 320. The third multiplexer 326 selects between the scan counter address values, SCAN_CNT [3:2] on line 312, generated for the Scan Dump mode or a fixed value for the scan row mode using the ATPG_MODE control signal on line 321. In this example, the scan row is set for row “0” in the ATPG_MODE which is 00′b binary.

The example read address multiplexer has an output multiplexer 332 operative to output the read address S_RA [1:0] on line 338. The output multiplexer 332 has an input 386 from a second multiplexer 334 and an input 337 from a third multiplexer 336. The output multiplexer 332 uses SCAN_CTL 323 to determine modes of operation and to select which address to output. The second multiplexer 334 selects between the functional input read address ra[1:0] and MBIST generated read address mbra[1:0] using the MB_RUN 320 control. The third multiplexer 336 selects between the scan counter address values, SCAN_CNT [3:2] on line 312, which are generated by the scan bit-cell counter 370 for the Scan Dump mode or a fixed value for the scan row mode using the ATPG_MODE 321 control. In this example, the scan row is set to 0, which is 00′b binary. Scan Dump mode uses the scan bit-cell counter 370 and will start counting when the SCAN_EN 290 control is high.

A scan bit-cell counter 370 provides for the Scan Dump mode to strobe row addresses in sequence to dump out the data in the array via the scan chain. A clock is input to increment the counter, a SCAN_EN 290 control will start the counter and the counter outputs are SCAN_CNT [3:2] on line 314 and SCAN_CNT [1:0] on line 312.

In this example, observation registers 380 and 385 capture addresses generated by the address selector. The observation registers 380, 385 are part of a scan chain (not shown) which samples combinatorial logic outputs from multiplexers 324 and 334, which output read/write addresses into the scan chain produced by the functional circuit on the device, or by the MBIST module. The observation registers capture the functional read/write addresses from lines 122 a and 142 a input to the multiplexers 324 and 334 so that the logic generating the addresses can be evaluated using the scan chain in ATPG/TT scan modes.

FIG. 6 is an example schematic that shows an example enable signal multiplexer 124 for the enable bus applied as input to the row clock circuit, such as that shown in FIG. 4, for the latch array. An enable scan multiplexer selects between the two inputs, functional read/write enable signal 124 a and MBIST generated enable signal 144 a. The output of the multiplexers is an enable signal bus represented by 124 b. One multiplexer is operative for each read enable and write enable circuit. The multiplexer 340 selects between the functional write enable 362 and the MBIST generated write enable 364 using MB_RUN 320. The multiplexer 350 selects between the functional read enable 352 and the MBIST generated read enable 354 using MB_RUN 320.

FIG. 7 is an example schematic depicting a scannable Latch Array 102 like that of FIG. 2, having a size 4×4 structure in the ATPG row “0” scannable mode. The ATPG row “0” scannable mode is active when inputs to the control block 510 SCAN_EN equals 1, ATPG MODE 321 equals 1, S_RE 358 is active and row 00′b 325 is set. In this mode, block 510 outputs a ROW_OUT control signal on line 256 operative to enable the row multiplexers row “0” data.

FIG. 7 shows an input pattern HGFE DCBA on the SCAN_IN line 130, which depicts abstractions of data, where ABCD, etc. are labels for a specific bit in the stream of bits going in. So, the letter A represents a bit having one of a binary 0 or binary 1 value at a specific location in the bit stream, as a way of showing the progression of that specific bit. The same is true for all the bits HGFE DCBA. For clarity, we use letter symbols rather than the binary values as way to track progression of that specific bit in the scan chain.

FIG. 7 shows ROW-0 106 after four scan cycles written with data ABCD, with letter “A” is in the A0 position, “B” is in the A1 position, “C” is in the A2 position, and “D” is in the A3 position. In a first cycle, data “A” is stored in the A3 latch, and fed back as input to the B2 latch. In a second cycle, data “A” is stored in the A2 latch, and data “B” is stored in the A3 latch, and so on. After four clocks, ROW-0 stores the data “D” in the A3 latch, “C” in the A2 latch, “B” in the A1 latch and “A” in the A0 latch. A fifth clock would shift the data with “E” entering the ROW-0 A3 latch and shifting “D” in the A2 latch, “C” in the A1 latch and “B” in A0 latch, the data “A” would then shift out via DOUT[0] to the SCAN_OUT line 160 via the buffer 234. This routing provides shift register movement of data through the selected row providing coverage of the peripheral circuits of the array (addressing, clocking, muxing . . . ) by the scan operation. After eight clocks, ROW-0 stores the data “H” in the A3 latch, “G” in the A2 latch, “F” in the A1 latch, and “E” in the A0 latch, and the data DCBA would be shifted down the scan chain.

FIG. 8 is a schematic block diagram using a representation of a scannable storage element array 586 comprising four 4×4 scannable latch array macros (“4r×4c”), each of which can be implemented as described with reference to FIG. 2. Concatenating the four 4×4 scannable latch array macros 102 results in a 4 row by a 16 column functional array, usable for example in a functional mode as a 16 bit vector FIFO. The representation includes 4r×4c LA_macro pin out list 584. Note that the terms WR_PTR [1:0] and RD_PTR [1:0] refer to read/write addresses pointers. The term “SI” is serial-in data, “SO” is serial-out or scan-out, “SE” is scan-enable. The schematic block diagram of a Verilog style block (la_wrapper 4r×16c 586) illustrates appending four 4×4 Latch Array macros (4r×4c LA_macro 584) to achieve a 4×16 Latch Array.

In some embodiments, the number of rows can be extended, and the number of columns is fixed for concatenation purposes. Using “mini” 4-column macro instantiations allows amortization of test logic and fixes the column count to “4”. Repeating 4 column macros can achieve a desired wrapper width where a byte is two macros, a word is four macros, etc.

The la_wrapper 4r×16c 586 illustrates additional signals, a Test at-speed mode with the signal “start”, a test static mode with the signal ATPG_MODE and a broadcast with Scan_En. There is also test static with daisy-chained SCAN_IN. MBIST input logic, staging flops, and Pass/Fail amortize over all 4r×4c macro instances.

In some embodiments, individual placement of each mini 4×4 Latch Array (4r×4c LA_macro 584) can be more efficient in a semiconductor integrated circuit as they are small units connected by scan stitching. Scan stitching between macros can allow for optimal place-and-route flexibility. This eases the problem of placement and routing as it is easier to place smaller chunks of Latch Array memory when building a FIFO or LUT (look-up table) unit. Timing constraints also impose restrictions. MBIST timing can require memory test circuits to be placed in proximity to MBIST controllers. The ability to split Latch Arrays into several macros allows for flexibility to meet timing constraints. Devices having a greater number of memory arrays spread over the same size (or wider) chip area can run into timing problems and additional routing complexities. This macro concept comes to solve that problem.

In some embodiments, Functional Mode implementations of the mini 4×4 Latch Array example (4r×4c LA_macro 584) are sized in row depth and column width for a FIFO operative to load data and pop data off a memory stack using (WR_PTR [1:0]) write addresses pointers and (RD_PTR [1:0]) read addresses pointers. In some embodiments, the system has flags for full, half-full and empty to monitor a FIFO memory stack. In other aspects, a FIFO wrap-around mode allows for circular buffering of data. In other embodiments, implementations of LUTs are used in the Functional Mode.

The following is an example pseudo-code method for a 4×4 Scan Dump shown.

-   Example Pseudo-Code Method for a 4×4 Scan Dump -   Step 0: Stop Chip, prepare for Scan Dump -   Stop clocks and set ATPG_MODE=0 -   Step 1: Select Scan Dump Mode -   Scan_EN=1 -   Row Counter points to row 0 -   Step 2: Begin scan unload -   4 shift clocks, Row 0 data shifts out scan-out -   Column Counter equals 2′11 increments Row Counter to 1 -   4 shift clocks, Row-1 data shifts out scan-out -   Column Counter increments Row Counter to 2 -   4 shift clocks, Row-2 data shifts out scan-out -   Column Counter increments Row Counter to 3 -   4 shift clocks, Row-3 data shifts out scan-out -   Column Counter increments Row Counter to 0 -   Repeat in other macros

FIG. 9A depicts a schematic showing an example 4×4 scannable Latch Array 102 loading row “0” with serial-in data, operative in Scan Dump mode. The decoder SCAN DUMP “A” LATCH ROW-MUXING 710 Row-0 mode is active when SCAN_EN equals 1, ATPG MODE 321 equals 0, S_RE 358 is active and SCAN_CNT [3:2] 312 equals 0 to output a ROW_OUT 256 control that enables the row muxes to select Row-0 for the ROW-DATA. An example Scan Dump mode shows an input pattern PONM LKJI HGFE DCBA that is the serial data for the SCAN_IN input on line 130 to the B3 latch. After four clocks, ROW-0 stores the data “D” in the A3 latch, “C” in the A2 latch, “B” in the A1 latch and A” in the A0 latch. In Scan Dump mode, the data “A” shifts out to the SCAN_OUT line 160 via the buffer 234 only after a wrap-around. This routing provides the shift register movement of data from each of the row outputs across the row-columns using clocking provided by the “A” latch row clocking circuit 126.

FIG. 9B depicts a schematic showing an example scannable Latch Array 102 loading row “1” with serial-in data, operative in Scan Dump mode with input TSRQ PONM LKJI HGFE as the serial data for the SCAN_IN input to the B3 latch remaining after scanning in DCBA. After eight clocks, ROW 1 stores the data “H” in the A3 latch, “G” in the A2 latch, “F” in the A1 latch, and E” in the A0 latch. The decoder SCAN DUMP “A” LATCH ROW-MUXING 711 ROW-1 mode is active when SCAN_EN equals 1, ATPG MODE 321 equals 0, S_RE 358 is active and SCAN CNT [3:2] 312 equals 01′b, and outputs a ROW_OUT 256 control that enables the row muxes to select Row-1 for the ROW-DATA [N:0] 245.

FIG. 9C depicts a schematic showing an example scannable Latch Array 102 loading row “2” with serial-in data, operative in Scan Dump mode. The decoder SCAN DUMP “A” LATCH ROW-MUXING 721 Row-2 mode is active when SCAN_EN equals 1, ATPG MODE 321 equals 0, S_RE 358 is active and SCAN CNT [3:2] 312 equals 10′b and can output a ROW_OUT 256 control to enable the row muxes to select Row-2 of the ROW-DATA [N:0] 245. The example Scan Dump mode shows an input TSRQ PONM LKJI on line 130 as serial data for the SCAN_IN input to the B3 latch. After four clocks, Row-2 stores the data “L” in the A3 latch, “K” in the A2 latch, “J” in the A1 latch and “I” in the A0 latch.

FIG. 9D depicts an example scannable Latch Array 102 loading row “3” with serial-in data operative in Scan Dump mode. The decoder SCAN DUMP “A” LATCH ROW-MUXING 721 mode is active when SCAN_EN equals 1, ATPG MODE 321 equals 0, S_RE 358 is active and SCAN_CNT [3:2] 312 equals 11′b and will output a ROW_OUT 256 control, enabling the row muxes to select row-3 for the ROW-DATA [N:0] 245. The input TSRQ PONM on line 130 is applied as the serial data for the SCAN_IN input to the B3 latch. After four clocks, ROW-3 stores the data “P” in the A3 latch, “O” in the A2 latch, “N” in the A1 latch and “M” in the A0 latch.

FIG. 9E depicts a schematic showing an example scannable Latch Array 102 wrapping row “0” with serial-in data, operative in Scan Dump mode and shifting a first row of data to SCAN_OUT==DCBA 730. The decoder SCAN DUMP “A” LATCH ROW-MUXING ROW-0 714 mode is active when SCAN_EN equals 1, ATPG MODE 321 equals 0, S_RE 358 is active and SCAN_CNT [3:2] 312 equals 00′b will output a ROW_OUT 256 control to enable the row muxes to select Row-0 for the ROW-DATA [N:0] 245. The input TSRQ on line 130 as the serial data for the SCAN_IN input to the B3 latch. After four clocks, ROW-3 stores the data “T” in the A3 latch, “S” in the A2 latch, “R” in the A1 latch, and “Q” in the A0 latch. Rows one, two, and three were previously filled in FIGS. 9A, 9B and 9C. During wrap-around mode, SCAN_OUT shifts the data DCBA after four clock cycles.

FIG. 10A illustrates a four row, four column scannable Latch Array after 17 scan shift clocks 812. The scannable Latch Array 810 first fills with sixteen data values, counting from a Bit 0 value to a Bit 15 value. On the 17th shift clock, upon wrap-around, the row-0 A0 column latch shifts out 812 the Bit 0 value to SCAN-OUT line 160 and shifts in the value of Bit 1. Bit 2 shifts into the row-0 A1 column latch, Bit 3 shifts into the row-0 A2 column latch and a Bit 16 shifts into the row-0 A3 column latch. The next bit value 17 811 is on the SCAN_IN line 130, ready to shift in on the next shift clock.

FIG. 10B illustrates a four row, four column scannable Latch Array after 31 scan shift clocks. The scannable Latch Array 820 fills with a second set of fifteen new data values, counting from a Bit 16 value to a Bit 30 value. On the 31st shift clock, during wrap-around, the value of Bit 15 shifts into the row-3 A0 column latch and the Bit 14 822 value shifts to SCAN-OUT line 160. Bit 28 shifts into the row-3 A1 column latch, Bit 29 shifts into the row-3 A2 column latch, and Bit 30 shifts into the row-3 A3 column latch. The next Bit value 31 821 is on the SCAN_IN line 130 ready to shift in on the next shift clock.

FIG. 10C illustrates a four row, four column scannable Latch Array after 32 scan shift clocks. The scannable Latch Array 830 first fills with a second set of fifteen new data values counting from a Bit 16 value to a Bit 31 value as shown in FIG. 11B. On the 32nd clock, during wrap-around, the row-3 A0 column latch shifts out the Bit 15 832 to SCAN-OUT line 160 and Bit 28 shifts into the row-3 A0 column latch. Bit 29 shifts into the row-3 A1 column latch, Bit 30 shifts into the row-3 A2 column latch, and a Bit 32 shifts into the row-3 A3 column latch. The next Bit value 32 831 is on the SCAN_IN line 130 ready to shift in on the next shift clock.

Testability of memory on integrated circuits is improved by connecting storage elements like latches in memory to scan chains and configuring memory for scan dump. The use of latches and similar compact storage elements to form scannable memory can extend the testability of high-density memory circuits on complex integrated circuits operable at high clock speeds. A scannable memory architecture includes an input buffer with active low buffer latches, and an array of active high storage latches, operated in coordination to enable incorporation of the memory into scan chains for ATPG/TT and scan dump testing modes. 

What is claimed is:
 1. A scannable memory circuit on an integrated circuit having a scan chain, comprising: an array of storage elements having a plurality of rows and N columns of storage elements (where N is a positive integer greater than 1); an input buffer including N buffer latches, each buffer latch of the N buffer latches respectively coupled to storage elements of one of the N columns in the array of storage elements, the N buffer latches enabled to load data during a first part of an input clock period, and configured to transfer data during a second part of the input clock period from the input buffer in parallel to storage elements of a write row in the plurality of rows in the array of storage elements, wherein the write row is selected by a write row address and storage elements of the write row are enabled to load data during the second part of the input clock period; an output selector having inputs coupled to outputs of the storage elements in the array of storage elements and controlled by a read row address to output data from storage elements in a read row in the plurality of rows in the array of storage elements in parallel to an output bus DOUT[N-1:0] having N lines; an input data selector having an output DIN[N-1:0] with N lines coupled to inputs of the N buffer latches in the input buffer, selecting in dependence on a scan enable signal from a first input connected to a functional data path, and a second input connected to a scan mode input bus, the scan mode input bus having a scan-in line connected to the scan chain for connection to DIN[N-1] and a plurality of feedback lines DOUT[N-1:1] from the output selector for connection to DIN[N-2:0], respectively; and a circuit, responsive to the scan enable signal to connect DOUT[ ] from the output selector to a scan-out line connected to the scan chain.
 2. The scannable memory circuit of claim 1, wherein the first part of the input clock period is between a falling edge and a rising edge of an input clock signal, and the second part of the input clock period is between a rising edge and a falling edge of the input clock signal.
 3. The scannable memory circuit of claim 1, wherein the storage elements in the array of storage elements respectively comprise a single transparent latch.
 4. The scannable memory circuit of claim 1, wherein a buffer latch of the N buffer latches comprises a first transparent latch and a storage element in the write row comprises a second transparent latch, and the first transparent latch and the second transparent latch are configured to act as a master-slave flip-flop in the scan chain.
 5. The scannable memory circuit of claim 1, wherein the storage elements in the array of storage elements respectively comprise static random-access memory (SRAM) bit cells.
 6. The scannable memory circuit of claim 1, including a control circuit coupled to the scan enable signal, an input clock signal, and the write row address to generate a row clock signal for storage elements in the write row during a scan operation shifting data from the scan-in line through the write row to the scan-out line.
 7. The scannable memory circuit of claim 1, including an address generator having an address output for access to the array of storage elements during a functional mode, and including a scan chain register connected to the address output of the address generator for registering addresses generated in the functional mode for observation accessible via the scan chain.
 8. The scannable memory circuit of claim 1, including an address selector responsive to a mode control input to select the read row address from a first input connected to functional circuits or a second input connected to a test address.
 9. The scannable memory circuit of claim 8, wherein the test address is a fixed row number for a scan mode or is generated by a counter for a scan dump mode or a memory test mode.
 10. The scannable memory circuit of claim 1, including: an address selector responsive to a mode control input to select the write row address from a first input connected to functional circuits or a second input connected to a scan mode address; and a latch row clocking circuit which receives the write row address from the address selector and generates row clock signals for storage elements of the write row that are active during the second part of the input clock period.
 11. The scannable memory circuit of claim 10, the address selector having a third input connected to an address counter for a scan dump mode.
 12. The scannable memory circuit of claim 10, the address selector having a third input connected to an address counter for a memory test mode.
 13. The scannable memory circuit of claim 10, including a register to capture addresses generated by the address selector, the register linked in the scan chain and off a data path of the selected addresses to the latch row clocking circuit.
 14. An integrated circuit comprising: an array of storage elements arranged into rows and columns, storage elements in the array of storage elements having respective data inputs, respective store enable inputs, and respective data outputs, the array of storage elements including a first column of storage elements, one or more other columns of storage elements, and a second column of storage elements; an input row buffer comprising a plurality of buffer latches having respective latch inputs and respective latch outputs and configured to load data during a first part of a period of an input clock, the plurality of buffer latches including a first buffer latch with its respective latch output coupled to the data inputs of each storage element of the first column of storage elements, one or more other buffer latches with their respective latch outputs coupled to the data inputs of each storage element in a corresponding column of the one of more other columns of storage elements in the array of storage elements, and a second buffer latch with its respective latch output coupled to the data inputs of each storage element of the second column of storage elements; a control circuit coupled to the input clock and a scan enable input and having a plurality of row clock outputs coupled to the store enable inputs of the storage elements of the array of storage elements, the control circuit configured to drive a row clock output of the plurality of row clock outputs coupled to storage elements of a single row of the array of storage elements to be active during a second part of the period of the input clock in response to the scan enable input being active; an output selector having inputs respectively coupled to the data outputs of the storage elements of the array of storage elements and outputs coupled to an output bus, the output selector configured to transfer data from the data outputs of a selected row of the array of storage elements to the output bus dependent upon a read address, the output bus including a second output line of the output bus corresponding to the second column of storage elements, and a set of output lines consisting of a first output line of the output bus corresponding to the first column of storage elements, and one or more other output lines of the output bus respectively corresponding to the one or more other columns of storage elements; a row of input selectors selecting, based on the scan enable input, between first inputs coupled to respective lines of an input bus and second inputs, the row of input selectors including a first input selector having an output coupled to the latch input of the first buffer latch and corresponding to the first column of storage elements, and a set of input selectors consisting of one or more other input selectors having outputs coupled to the respective latch inputs of the one or more other buffer latches and respectively corresponding to the one or more other columns of the array of storage elements, and a second input selector having an output coupled to the latch input of the second buffer latch and corresponding to the second column of storage elements, the second input of the first input selector coupled to a scan input line, and the second inputs of the set of input selectors individually coupled to a line selected from the set of output lines, wherein each line of the set of output lines is coupled to an input selector corresponding to a different column of the array storage elements than a column corresponding to that line.
 15. The integrated circuit of claim 14, including a circuit, responsive to the scan enable signal, to connect the second output line of the output bus to a scan output line; wherein the scan input line, the input row buffer, at least one row of storage elements of the array of storage elements, and the scan output line are a part of a scan chain during a scan operation indicated by the scan enable signal and the input selectors in the row of input selectors are responsive to the scan enable signal to select the second inputs.
 16. The integrated circuit of claim 14, wherein the first column of storage elements holds a most significant bit of a data element, the second column of storage elements holds a least significant bit of the data element, and each line of the set of output lines is coupled to an input selector corresponding to a column of the array of storage elements that is adjacent to a column of the array of storage elements corresponding to the respective line of the set of output lines.
 17. The integrated circuit of claim 14, further comprising: a write address; a write enable signal; the control circuit further configured to drive a row clock output of the plurality of row clock outputs to be active for a row of the array of storage elements, determined by the write address, during the second part of the period of the input clock in response to the write enable signal being active.
 18. The integrated circuit of claim 17, further comprising: a test pattern generator to generate the write address and a test pattern on data outputs; a data input selector having first inputs connected to functional circuits and second inputs connected to the data outputs of the test pattern generator, the data input selector having outputs coupled to the first inputs of the row of input selectors.
 19. The integrated circuit of claim 17, including: an address selector responsive to a mode control input to select the write address from an address provided by functional circuits, a fixed scan mode row address, a scan dump counter, or a test pattern generator; the control circuit further configured to drive the row clock output of the plurality of row clock outputs to be active for a row of the array of storage elements, determined by the write address, during the second part of the period of the input clock in response to the scan enable input being active.
 20. The integrated circuit of claim 14, wherein the buffer latches in the plurality of buffer latches have active low latch enable inputs and the store enable inputs of the storage elements in the array of storage elements are active high.
 21. The integrated circuit of claim 14, including an address selector responsive to a mode control input to select the read address from a first input connected to functional circuits or a second input connected to a test address.
 22. The integrated circuit of claim 21, wherein the test address is a fixed row number for a scan mode or is generated by a counter for a scan dump mode or a memory test mode. 